Inkjet printing devices having a through-flow print head are known in the art, e.g. from WO 2006/030235 A2 and WO 2006/064036 A1. In print heads having a through-flow arrangement fluid is removed continuously from the nozzle(s) in order to remove dirt and air bubbles that might block the nozzle or otherwise might affect a correct operation. Also heat generated by the electronic components of the print head, for example the piezo transducer used for generating drops of fluid, is removed, thereby conditioning the temperature of the fluid in the print head, which is significant as the fluid viscosity and consequently jetting properties of the fluid are dependent from the temperature. Inkjet printing heads are designed for either continuous drop generation or drop generation on demand. In drop-on-demand inkjet printing drops of fluid are only ejected from the respective nozzle(s), when such a drop is required for printing the substrate, contrary to continuous systems where a continuous stream of fluid drops is generated, a fluid drop being deflected to the substrate when it is required, while the remaining drops are collected. Drop-on-demand inkjet printing systems are usually further classified according to the drop generation principle, either thermal or piezo-electrical.
In inkjet print heads a slightly negative pressure or back pressure is required for operation. WO 2006/030235 A2 and WO 2006/064036 A1 both disclose fluid supply and circulation systems for use in inkjet printing devices having through-flow print heads, wherein the back pressure is controlled by active control of the pressures in a supply subtank supplying fluid to the nozzle of the through-flow print head and in a return subtank receiving fluid not consumed by the print head. The return subtank is connected to a main reservoir, from which the supply subtank is fed.
The subtanks and associated conduits contain a substantial volume of fluid, e.g. about 10 ml per print head. Upon interruption of a printing job, e.g. at the end thereof or because of temporarily failure, there is the risk of leaking fluid from these subtanks and associated conduits via the nozzle into a collecting tray or the like, because the slight negative pressure at the nozzle disappears, eventually resulting in almost complete emptying of the subtanks and associated subtanks. This risk is significant in inkjet printing devices, where the pressures in the subtanks is not actively monitored and controlled. The amount of fluid thus collected, which is to be disposed off as waste, could be relatively large. Disposal of valuable fluid adds to the costs. Furthermore restarting the device might be difficult.